This invention relates to processes and plants for the electrodeposition of metals and, more particularly but not exclusively, is concerned with the extraction and recovery of cobalt and other metals from their ores.
There are known to us extraction and recovery processes which involve the leaching of cobalt into solution followed by purification of the cobalt-rich solution and precipitation of cobalt as one of its salts or oxides. In one such process, the cobalt is precipitated as its basic oxide, and the precipitate is dissolved in an electrolyte of a cobalt electrowinning circuit at a rate sufficient to replace the cobalt removed from the electrolyte during the electrowinning process (this operation frequently being referred to as "the re-solution stage"). The coulombic efficiency of cobalt electrodeposition at the cathode of an electrolytic cell is greatly influenced by the concentration of acid present in the cobalt electrolyte and, in conventional electrowinning processes, falls with increasing concentration of acid. In existing processes only about 5 gpl (gms per liter) cobalt can usually be removed efficiently from solution per pass through the electrowinning cell because the electrodeposition of cobalt releases acid into the solution which adversely affects the coulombic efficiency. Electrowinning is frequently carried out from solutions containing about 40 gpl cobalt and it is readily seen that the small "delta cobalt", i.e. the small concentration change, in the electrowinning stage, inherent in the above process gives rise to a high recycling load of cobalt in the solution and high flow rates through the plant. There must be acid present in the electrolyte at the re-solution stage for there to be dissolution of the basic cobalt oxide. However, it is not feasible to retain in the electrolyte the acid generated in the cell while operating with a high "delta cobalt" in the electrolyte, i.e. a large concentration change, in the electrowinning process.
Typical examples of the processes outlined above are described in "World Mining", September 1970, pages 42-47.
It has also been proposed to precipitate the cobalt as its sulphate and in such a process the feed precipitate cannot neutralise the acid and it is therefore essential that the acid generated in the electrowinning cell be removed from the electrowinning circuit.
Although the above description relates exclusively to the electrowinning of cobalt, it will be apparent that similar considerations apply to the extraction and recovery of a number of other metals, for example nickel and zinc, by the known electrowinning processes.
Our copending British Patent Application No. 31524/74 describes and claims an electrolytic process, for the electrodeposition of a metal from an aqueous solution of a salt of said metal, which process comprises the steps of disposing between the anode and cathode of an electrochemical cell a separator which incorporates an anion exchange membrane which is substantially impermeable to cations, so as to form separate anode and cathode compartments within said cell, establishing within said cathode compartment a particulate cathode, flowing said aqueous solution into said cathode compartment, imposing a potential difference across the anode and cathode of said cell sufficient to electrodeposit metal from said aqueous solution of a salt of said metal on to the particulate cathode, and allowing passage of anions through said anion exchange membrane. The said patent application also describes and claims in electrochemical cell, suitable for use in the electrodeposition of metal from an aqueous solution of a salt of said metal, wherein the cell is provided with a separator which is disposed between the cathode and anode of the electrochemical cell so as to form separate anode and cathode compartments within said cell and which incorporates an anion exchange membrane, and wherein the cathode compartment contains a particulate cathode.
The electrolytic process and electrochemical cell described and claimed in the above mentioned patent application enable the electrowinning of cobalt to take place without a number of disadvantages associated with conventional cobalt electrowinning practice. For example, cobalt is conventionally deposited on stainless steel "blank" cathodes. After the deposit has grown to an acceptable thickness it is stripped from these blank cathodes. The removal of the deposit from the blanks can be an arduous process because in some cases the cobalt metal adheres strongly to the stainless steel blank. The removal of deposited cobalt from the blank is usually carried out manually using hammers or chisels. This can lead to damage of the blanks. These problems are obviated by the electrolytic process and electrochemical cell described and claimed in the said patent application. Nevertheless, problems associated with the production of acid during the electrolytic process are not entirely overcome by our earlier process and cell. Accordingly, it is an object of the present invention to provide an electrolytic process and electrolytic cell in which the problems associated with the production of acid during the electrowinning of, inter alia, cobalt are ameliorated.